This invention relates to an atmospheric RF/DC device, specifically to such RF/DC devices which are used for analyzing gas-phase ions at atmospheric pressure.
The analytical utility of a RF/DC (radio frequency/direct current) mass filter or analyzers, such as a quadrupole mass filter, as a device for continuous selection and separation of ions under conventional vacuum conditions is well established. It also has a highly developed theoretical basis (1, 2, 3, 4, 5, 6). The desirable performance attribute of the quadrupole mass filter is the fact that motion in the x, y, and z directions are decoupled, (i e. motion in each direction is independent of motion of the other directions in the Cartesian coordinate system) (7). In general, a time varying potential is applied to opposite sets of parallel rods as illustrated in FIG. 1.
The xe2x80x9chyperbolicxe2x80x9d geometry in the x-y plane coupled with the appropriate time-varying applied potential (an RF field) creates a pseudo-potential well that will trap ions within a xe2x80x9cstablexe2x80x9d mass range along the centerline of the x-y plane (the z-axis), while ejecting ions of xe2x80x9cunstablexe2x80x9d mass in the x and y directions. In a quadrupole operated a low pressures (under vacuum,  less than 10xe2x88x923 torr), motion along the z-axis is generally determined by the initial energy of the ions as they enter the quadrupole field, and can be generally considered equivalent to motion in a field free environment. One notable exception to this field-free model would be the effects the fringing fields at the entrance and exit of the quadruple. At the entrance and exit from quadrupoles the x, y and z motions are coupled. This results in the transfer of small amounts of translational energy between the different dimensions. The effects of which can generally be reduced dramatically through electrode design (e.g. the use of RF-only pre- and post-filters).
Ion motion within a quadrupole is well characterized, and is described by the various solutions of the Mathieu equation (8). Simply stated, for a given ion with a particular mass-to-charge ratio (m/z), there exist sets of RF (alternating at the radio frequency) and DC (direct current) voltages, which when applied to a quadrupole yield stable trajectories. These sets of RF and DC voltages can be plotted to represent regions of stability both in the x and y directions (as shown in FIG. 2A). Since motion in the x and y directions are de-coupled, it is convenient to plot both directions in a single plot, focusing on the region(s) where stable trajectories are possible simultaneously in both the x and y directions. This region of stability is designated the xe2x80x9cbandpass regionxe2x80x9d.
According to the analytical theory based on the Mathieu equation, any set of voltages which do not lie within one of these regions of stability (in both x and y directions) will result in an unstable trajectory of ions, with exponentially increasing acceleration from the centerline of the quadrupole in the instable direction (x or y). These stability boundaries tend to be very sharp, and can therefore be used to reject certain masses while accepting other masses. Since each mass has a unique set of stable voltages, judicious selection of voltages can allow selection of a narrow bandpass of masses to be transmitted through the quadrupole at the expense of all others as illustrated in FIG. 2B. Quadrupole mass spectrometers are typically scanned through the mass range by increasing both RF and DC voltages while maintaining a constant ratio (see xe2x80x9cScan Linexe2x80x9d in FIG. 2B). The slope of the scan line determines the resolution of the mass spectrometer.
There is evidence that these stability boundaries observed with convention quadrupole operation are independent of the operating pressure, and therefore that mass resolution should be possible even for a quadrupoles operated at higher pressures, such as atmospheric pressure. The majority of research with higher pressures has occurred in the pressure range of 1xc3x9710xe2x88x925 to 1xc3x9710xe2x88x921 torr with the three-dimensional quadrupole ion trap (9, 10). It has been clearly observed with three-dimensional quadrupole ion traps that stability boundaries may actually be sharpened at these higher pressures yielding improved resolution. But there are limits with the operating pressures. As the pressure is increased in quadrupole devices the incidence of a gas discharge increases as illustrated in recent studies of ion pipes by Bruce Thomson and coworkers (11).
FIG. 3 illustrates that there are two pressure regimes where time-varying fields can be established at sufficient field strength to affect the radial displacement of unstable ions; the first is at low pressures ( less than 10xe2x88x922 torr, where existing quadrupole mass analyzers are operated) and the second is at atmospheric pressure (100-760 torr, the present invention). The region marked forbidden at intermediate pressures is limited by gas discharge at the higher voltages (or fields) required for quadrupole mass filtering. In addition, scattering effects from discrete collisions between ions and the surrounding gases deleteriously affect the motion of the ions in the intermediate pressure region as well.
In recent years ion mobility spectrometry (IMS) has become an important analytical tool for measurement of ionized species created in a wide variety of atmospheric pressure ion sources; including, discharge, 63Ni, and photo-ionization. (12, 13) Recently, a number of researchers have also incorporated the LC/MS type sources of electrospray (ES) and atmospheric pressure chemical ionization (APCI) into IMS. (14, 15, 16, 17)
One recent non-conventional implementation of IMS (known as FAIMS, high-field asymmetric waveform ion mobility spectrometry) utilizes an asymmetric waveform to isolate ions between parallel plates or concentric tubes. (18, 19) This technique demonstrates the principal that we propose with the present invention, in that it utilizes a flow of gas along the z-axis coupled with alternating field conditions to create a bandpass spectrometer. Of particular note is the ability to produce field strengths of well over 10,000 volts per cm without discharge occurring. When coupled to ES and mass spectrometry FAIMS has served as an effective means of fractionation of various molecular weight regimes (20).
Nevertheless all the RF/DC mass filters, linear and three-dimensional quadrupoles and FAIMS heretofore known suffer from a number of disadvantages:
(a) Conventional quadrupole mass analyzers require vacuum components; namely, vacuum chambers, high-vacuum electrical feed-throughs, sealed pumpout lines, gauges and others expensive vacuum related devices that can withstand large pressure differences (up to 1000 torr). This requires sufficiently strong materials such as stainless steel, aluminum, or other vacuum compatible materials; chambers with vacuum tight welds; or metal or rubber seals, all with little or no outgassing.
(b) Conventional quadrupole mass analyzers require expensive high vacuum pumps, such as turbomolecular or diffusion pumps; and low vacuum pumps, such mechanical vane pumps; costing many thousands of dollars. The cost of these pumps can makeup approximately 20% of the total cost of an instrument.
(c) Atmospheric interfaces for quadrupole mass analyzers can require multiple stages of rough pumping and expensive high vacuum pumps for operation, resulting in costly and complex interface designs.
(d) Quadrupole mass analyzers weight several hundred pounds and require a substantial amount of electrical power for operation, heating and cooling, etc.; all restricting their portability.
(e) These all add to the manufacturing cost of a quadrupole mass spectrometer thereby resulting in a large percentage ( greater than 50%) of the cost of a mass analyzer being due to the cost of the vacuum system components, including the vacuum pumps (both high and low vacuum), chamber, vacuum feed-throughs; atmospheric pressure interfaces; etc.
(f) FAIMS lack the precision and band pass capabilities of quadrupolar designs or other multi-pole designs, by only utilizing 2 parallel plates instead of multiple poles. In essence by utilizing asymmetric RF voltages between parallel plates FAIMS is forming only one-half of the fields seen in quadrupolar designs, therefore stopping short of the precision and band-pass capabilities of quadrupolar devices.
(g) FAIMS""s present design suffers from a very inefficient sampling of atmospheric gas-phase ions into the area between the parallel plates.
In accordance with the present invention an atmospheric or near atmospheric RF/DC mass analyzer comprises an atmospheric ion source, an ion-focusing region, an RF/DC quadrupole, an atmospheric gas-phase ion detector, and a source of gas.
Accordingly, besides the objects and advantages of conventional quadrupole mass analyzers described in the previous sections, several objects and advantages of the present invention are:
(a) to provide a RF/DC mass analyzer that can be produced in a variety of materials without requiring the need for materials and/or construction that can withstand large pressure difference and sealing associated with vacuum devices;
(b) to provide a RF/DC mass analyzer which does not require the use of high vacuum pumps;
(c) to provide a RF/DC mass analyzer which does not require high vacuum pumps for atmospheric pressure ion-source interfacing;
(d) to provide a RF/DC mass analyzer which both is lightweight and portable;
(e) to provide a RF/DC mass analyzer whose production allows both for an inexpensive and easily mass produced RF/DC device;
(f) to provide a RF/DC mass analyzer which can provide a precise band-pass capability;
(g) to provide a RF/DC mass analyzer which can efficiently sample gas-phase ions at atmospheric pressure.
Further objects and advantages are to provide an atmospheric RF/DC mass analyzer which can be composed of plastic and other easily molded or composit materials; the rods can be solid, tubes, or make of perforated metal sheets; ion source can be an atmospheric pressure ionization source; such as electrospray, atmospheric pressure chemical ionization, photo-ionization; corona discharge; inductively coupled plasma source, etc.; or ion detector can be an active pixel sensor array. Still further objects and advantages will become apparent for a consideration of the ensuing descriptions and drawings.
The lack of vacuum requirement for the present device will enable the present spectrometer to be fabricated with a wide variety of fabrication alternatives not readily available with vacuum devices, such as micro-machining, micro-lithography for lenses and element, lamination, and molding. The result being a less expensive, smaller, lighter, and more portable detection device.
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